A transformer comprising orthogonal windings is previously known from U.S. Pat. No. 4,210,859, to Meretsky et al. of Apr. 18, 1978 (hereinafter “Meretsky”). However, the known solution manifests several disadvantages. Some of these disadvantages are described below.
In general, the problem with the prior art as illustrated by Meretsky is that it does not present a complete picture of how the manipulation of the domains with a DC control current affects the magnetisation in relation to the connection between two orthogonal windings. In Meretsky, a device is described which is developed on the basis of a test conducted on a ferrite pot core with dimensions 18×11 mm, and with current levels in the mA range. Ferrite, however, is not suitable for use at high power levels, for example, because of the high material costs associated with it. The high costs limit the size of a ferrite core from the production engineering point of view. Further, higher power levels can be transferred by increasing the frequency of the voltage that has to be converted, but this requires complicated and expensive power electronics.
Meretsky illustrates a connection diagram for a variable transformer solution with 4 windings: a primary main winding, a secondary main winding arranged at a right angle to the primary winding, and two control windings, one for each main winding. The mode of operation is such that a variable DC current in both control windings will result in a transfer of AC voltage from the primary winding to the secondary winding. A transformer of this kind cannot be considered a realistic option, particularly if it is to be applied outside the mA range, because a DC current in the control windings will rotate the domains in the magnetic material in an unfavourable direction for connection in one half cycle of the primary voltage. These domain rotations cause harmonics in the secondary voltage. This phenomenon, is not taken into consideration in Meretsky.
In order to be able to implement a realistic solution for a variable power transformer, the problem arises that the control winding on the primary side is transformatively connected to the primary winding and will be under voltage from the primary side, thereby making it very difficult to regulate without extensive filtering.
Meretsky also discloses a transformer connection (FIG. 18) where windings with right-angled axes are interconnected in series two by two. The publication states that the core's utilisation can be increased by using such a connection. This is not correct, however, since the magnetic fields for the windings are summed vectorially and the described effect will not be achieved.
Meretsky also describes (FIG. 20) a variable delay between the input and output voltage in a case where the control windings each carry current and are interconnected in series. Phase distortion is involved here since the fields through the primary and the secondary winding are shifted via the domain directions. With the control windings connected in this manner, the device will not work for a power transformer used as a phase inverter, since the connection from the primary winding will influence the control current to such an extent that in principle the same connection as mentioned earlier (FIG. 18) will be obtained.